BY FORM FACTORS. 91 



h is ascertained by cutting off a piece from the top downwards, 

 and repeating the operation, until the point has been ascer- 

 tained where the basal area contains n concentric rings. 



Then the basal area s at ~ is ascertained, either by making a 



JL 



cross section at the spot, or by ascertaining the breadth of the 

 last n rings with the increment borer, thus ascertaining the 

 diameter which the tree had n years ago. In either case 

 the diameter increment must be measured in several places of 

 the circumference, so as to obtain the mean. 



-H 



In order to simplify the operation, Pressler proposed to cut 

 off a length corresponding to n years height growth in the 

 first place, and then to measure the basal area in both cases at 



g. He obtains the increment according to the formula : 



I=Sxh-sxh=(S-s) h. 

 The error due to omitting the top is said to be compensated 



TT 



for by S having been taken somewhat below ^-. 



a 



iii. DETERMINATION OF INCREMENT BY FORM FACTORS. 

 Let S be the basal area of a tree, taken at chest height ; s the 

 basal area of the tree n years ago taken at the same height ; 

 H and h the corresponding heights, and F and /the corres- 

 ponding form factors, then the increment : 



I=SxHxF-sxhxf. 

 In the case of a standing tree, H is measured with a height 



